1. Field of the Invention
This invention relates to a catalyst system which is a combination of at least one homogeneous catalyst and at least one heterogeneous catalyst. Changing the homogeneous catalyst used with the heterogeneous catalyst effects the molecular weight distribution of the resulting polymer.
2. Description of the Prior Art
It is known that two or more homogeneous catalysts, such as those based on metallocene compounds, may be combined to effect properties, such as molecular weight distribution. U.S. Pat. No. 4,530,914 discloses use of a catalyst system comprising two or more metallocenes in the polymerization of .alpha.-olefins, primarily ethylene, to obtain a broad molecular weight distribution. The metallocenes each have different propagation and termination rate constants. The metallocenes are mixed with an alumoxane to form the catalyst system.
It is also known that metallocenes may be affixed to a support to simulate a heterogeneous catalyst. In U.S. Pat. No. 4,808,561 discloses reacting a metallocene with an alumoxane and forming a reaction product in the presence of a support. The support is a porous material like talc, inorganic oxides such as Group IIA, IIIA IVA OR IVB metal oxides like silica, alumina, silica-alumina, magnesia, titania, zirconia and mixtures thereof, and resinous material such as polyolefins like finely divided polyethylene. The metallocenes and alumoxanes are deposited on the dehydrated support material.
In U.S. Pat. No. 4,701,432 a support is treated with at least one metallocene and at least one non-metallocene transition metal compound. To form a catalyst system a cocatalyst comprising an alumoxane and an organometallic compound of Group IA, IIA, IIB and IIIA is added to the supported metallocene/non-metallocene. The support is a porous solid such as talc or inorganic oxides or resinous materials, preferably an inorganic oxide, such as silica, alumina, silica-alumina, magnesia, titania or zirconia, in finely divided form. By depositing the soluble metallocene on the support material it is converted to a heterogeneous supported catalyst. The transition metal compound, such as TiCl.sub.4, is contacted with the support material prior to, after, simultaneously with or separately from contacting the metallocene with the support.
An advantage of a homogeneous (metallocene) catalyst system is the very high activity of the catalyst and the narrow molecular weight distribution of the polymer produced with a metallocene catalyst system. The metallocene catalysts suffer from a disadvantage in that the ratio of alumoxane cocatalyst to metallocene is high, requiring extensive treatment of the polymer product to remove the aluminum. Another disadvantage of the homogenous catalyst system is that the polymer product has small particle size and low bulk density. Another disadvantage of the homogeneous catalyst system is that the reactor fouls during polymerization.
It would be advantageous to provide a catalyst system which facilitates use of a homogeneous catalyst but eliminates the disadvantages of such a system. A combination of a homogeneous catalyst with a heterogeneous catalyst in a single reactor will eliminate the disadvantages of the homogeneous catalyst alone and will facilitate use of a homogeneous catalyst. A combination of a homogeneous catalyst with a heterogeneous catalyst in a single reactor will also provide a means to control molecular weight distribution and polydispersity.